hammer2 - Refactor frontend part 7/many

hammer2 - Refactor frontend part 7/many

* Start fleshing out the XOP infrastructure.  The frontend will use this
  infrastructure to replicate VOPs to per-node threads for cluster operation.

  The basic idea here is for a VOP to allocate an xop and throw it into a
  xop_group (of which several are available).  ALL nodes associated with
  that xop_group will then process the xop each in their own thread.

  The xop can be detached from the frontend once the cluster operation
  completes, if interruptability is desired due to a stalled cluster,
  or once the operation has run on a sufficient number of nodes but still
  might be running or stalled on remaining nodes.

* Rename hammer2_syncthr.c, it is now more generic.